Fatty acid synthetase, a multienzyme complex, purified to homogeneity from lactating bovine mammary gland, has a relatively strong TPNH-dependent crotonyl-CoA reductase activity. The reductase activity is an inherent property of the enzyme and is believed to be a property of the enzyme from mammalian liver of other species and mammary glands The objective of the investigation is to deterine whether the site of reductase activity is the same as that of the synthesis of fatty acids. If so, the investigation will be directed towards determining whether or not the crotonyl group of crotonyl-CoA has to be transferred to the 4' phosphopantetheine prosthetic group of the enzyme before reduction by TPNH.